An image forming apparatus utilizing a digital technique, such as an electronic copying machine, is equipped with a fixing apparatus for fixing, to a sheet of paper, an image of a melted developer by pressure.
The fixing apparatus comprises a heating member for melting a developer, such as toner, and a pressure member for applying a predetermined pressure to the heating member, a predetermined contact width (nip width) being defined between the contact region (nip portion) of the heating and pressure members. When a sheet of paper with an image of a developer melted by the heating member is passed through the nip portion, the image is fixed on the sheet by pressure from the pressure member.
As a method for heating a heating member using induction heating, it is known to generate a magnetic field by an exciting coil and apply it to a roller-shaped heating member that has an outer periphery formed of a conductive thin film, thereby generating therein an eddy current for heating. In this case, to maintain the rigidity, the conductive thin film is formed of a metal (e.g., nickel).
However, metal films are liable to be oxidized. When the metal film of the heating member, which is formed of, for example, nickel, is heated to 200° C. or more and further pressed by a pressure member, it is degraded due to thermal fatigue and hence cannot be maintained in function over a long period. Further, when it is cooled after heating, the metal film of the heating member may well be broken due to thermal hysteresis, which makes it difficult to maintain the performance of the heating member for a long time.
Further, a heating member is known which has a nickel film formed by electroplating. In this case, thermal degradation of nickel is conspicuous near 200° C., which is a great restriction in use. In addition to this, plating causes problems in manufacturing, such as an inevitable increase in the scale of equipment, difficulty in management of the thickness of plating, and environmental problems concerning liquid wastes.
There is a need for a heating member with a conductive film free from the above problems.